For measuring a target point, numerous geodetic measuring devices have been known since ancient times. In this case, direction or angle and usually also distance from a measuring device to the target point to be measured are recorded and, in particular, the absolute position of the measuring device together with reference points possibly present are detected as spatial standard data.
Generally known examples of such geodetic measuring devices include the theodolite, tachymeter and total station, which is also designated as electronic tachymeter or computer tachymeter. One geodetic measuring device from the prior art is described in the publication document EP 1 686 350, for example. Such devices have electrical-sensor-based angle and, if appropriate, distance measuring functions that permit direction and distance to be determined with respect to a selected target. In this case, the angle and distance variables are determined in the internal reference system of the device and, if appropriate, also have to be combined with an external reference system for absolute position determination.
In many geodetic applications, points are measured by specifically configured target objects being positioned there. The latter usually consist of a plumb staff with a reflector (e.g. an all-round prism) for defining the measurement path or the measurement point. In the case of such measurement tasks, for controlling the measurement process and for defining or registering measurement parameters, a number of data, instructions, speech and further information are transmitted between target object—in particular a handheld data acquisition device at the target object—and central measuring device. Examples of such data include the identification of the target object (type of prism used), inclination of the plumb staff, height of the reflector above ground, reflector constants or measurement values such as temperature or air pressure. These information items or situation-governed parameters are necessary for enabling highly precise targeting and measurement of the measurement point defined by the plumb rod with prism.
Modern total stations have microprocessors for digital further processing and storage of detected measurement data. The devices generally have a compact and integrated design, wherein coaxial distance measuring elements and also computing, control and storage units are usually present in a device. Depending on the expansion stage of the total station, motorization of the targeting or sighting device and—in the case of the use of retroreflectors (for instance an all-round prism) as target objects—means for automatic target seeking and tracking can additionally be integrated. As a human-machine interface, the total station can have an electronic display control unit—generally a microprocessor computing unit with electronic data storage means—with display and input means, e.g. a keyboard. The measurement data detected in an electrical-sensor-based manner are fed to the display control unit, such that the position of the target point can be determined, optically displayed and stored by the display control unit. Total stations known from the prior art can furthermore have a radio data interface for setting up a radio link to external peripheral components such as e.g. a handheld data acquisition device, which can be designed, in particular, as a data logger or field computer.
For sighting or targeting the target point to be measured, geodetic measuring devices of the generic type have a telescopic sight, such as e.g. an optical telescope, as sighting device. The telescopic sight is generally rotatable about a vertical axis and about a horizontal tilting axis relative to a base of the measuring device, such that the telescopic sight can be aligned with the point to be measured by pivoting and tilting. Modern devices can have, in addition to the optical viewing channel, a camera for detecting an image, said camera being integrated into the telescopic sight and being aligned for example coaxially or in a parallel fashion, wherein the detected image can be represented, in particular, as a live image on the display of the display control unit and/or on a display of the peripheral device—such as e.g. the data logger—used for remote control. In this case, the optical system of the sighting device can have a manual focus—for example an adjusting screw for altering the position of a focusing optical system—or an autofocus, wherein the focus position is altered e.g. by servomotors. By way of example, such a sighting device of a geodetic measuring device is described in EP 2 219 011. Automatic focusing devices for telescopic sights of geodetic devices are known e.g. from DE 197 107 22, DE 199 267 06 or DE 199 495 80.
The optical system or the optical viewing channel of the sighting device usually contains an objective lens group, an image reversal system, a focusing optical system, a reticle for producing a reticle and an eyepiece, which are arranged e.g. in this order from the object side. The position of the focusing lens group is set depending on the object distance in such a way that a sharp object image arises on the reticle arranged in the focusing plane. Said image can then be viewed through the eyepiece or e.g. detected with the aid of a camera arranged coaxially.
By way of example, the construction of generic telescopic sights of geodetic devices is disclosed in the publication documents EP 1 081 459 or EP 1 662 278.
On account of the beam path that is usually to be utilized jointly both as viewing channel and for measurements, such devices require the technical design of said beam path in the manner of construction of a telescope with specialized, high-precision optical systems that are to be produced with a high outlay. Furthermore, an additional separate transmitting and receiving channel and also an additional image plane for the wavelength of the distance measuring device are provided for the coaxial electronic distance measurement.
Since target objects (e.g. the plumb rods with target mark, such as an all-round prism, which are usually used for geodetic purposes) can be targeted sufficiently precisely with the naked eye on the basis of the sighting device despite the 30-fold optical magnification often provided (i.e. not conforming to geodetic accuracy requirements), conventional measuring devices in the meantime have as standard an automatic target tracking function for prisms serving as target reflector (ATR: “Automatic Target Recognition”). For this, a further separate ATR light source—e.g. a multimode fiber output, which emits optical radiation having a wavelength in the range of 850 nm—and a specific ATR detector (e.g. CCD area sensor) sensitive to said wavelength are conventionally additionally integrated in the telescope.
In the context of the ATR fine targeting function, in this case the ATR measurement beam is emitted in the direction of the optical targeting axis of the sighting device and is retroreflected at the prism and the reflected beam is detected by the ATR sensor. Depending on the deviation of the alignment of the optical targeting axis from the prism, in this case the impingement position of the reflected radiation on the ATR sensor also deviates from a central sensor area positioning rate (i.e. the reflection spot of the ATR measurement beam retroreflected at the prism on the ATR area sensor does not lie in the center of the ATR area sensor and therefore does not impinge on a desired position defined e.g. on the basis of calibration as that position which corresponds to the optical targeting axis).
If this is the case, if the alignment of the sighting device is slightly readjusted in a motorized manner in such a way that the ATR measurement beam retroreflected at the prism impinges highly precisely in the center of the sensor area on the ATR area sensor (i.e. the horizontal and vertical angles of the sighting device are thus iteratively changed and adapted until the center of the reflection spot coincides with the desired position on the ATR area sensor). Alternatively, a residual deviation between the impingement point of the retroreflected ATR measurement beam on the ATR area sensor and the center of the sensor area can also be taken into account computationally and converted into an angle, which is correspondingly added to the solid angle—detected with the aid of the angle sensors—at which the targeting axis points. In other words, the solid angle with respect to the target point can in this case also be derived from the solid angle—detected with the aid of the angle sensors—of the targeting axis and an offset of the detected ATR measurement beam reflection from the sensor center (i.e. from that central point on the ATR sensor at which the targeting axis is imaged).
As a result, the achievable accuracy in the alignment of the optical targeting axis with the prism can be significantly increased by comparison with manually performed targeting with a reticle and on the basis of measurement by the naked eye. In order to ensure the functioning of the automatic targeting on the basis of evaluation of the position of the reflection spot of the ATR measurement beam retroreflected at the prism on the ATR area sensor, it is necessary, before the function starts, to align the sighting device with the target reflector at least approximately in such a way that the retroreflected ATR measurement beam also impinges on the ATR area sensor. For this purpose, it is possible e.g. beforehand to effect manual targeting of the target reflector on the basis of measurement by eye or to perform an automatic coarse targeting function.
The manual, coarse sighting of the target object can be effected by the user, on the one hand, by viewing and targeting the target object on a user display of the display control unit directly on the measuring device or on the display of a separate peripheral device (e.g. data logger as remote control). Often, however, this is still effected by means of viewing the target through the eyepiece of the telescopic sight (i.e. of the sighting device), since a displayed display image on the display control unit or the data logger may be insufficiently recognizable during use in practice—e.g. in the case of insolation.
Besides the ATR fine targeting function, an automatic target tracking functionality can also be provided in a similar manner and using the same ATR components (such as ATR light source and ATR detector). After ATR fine targeting has been effected (i.e. once the sighting device is aligned with the target in such a way that the center of the ATR measurement radiation reflection spot coincides with the desired position—corresponding to the targeting axis—on the ATR area sensor), the sighting device can furthermore be tracked to movements of the target “live” and appropriately rapidly in such a way that the center of the ATR measurement radiation reflection spot furthermore remains as accurately as possible and always on the desired position on the ATR area sensor. It is then often stated that the target is “locked on”. Problems can occur here if the target moves so jerkily and rapidly that it disappears from the field of view of the ATR detector (i.e. ATR measurement radiation reflected at the target no longer impinges on the ATR area sensor).
By way of example, EP 2 141 450 describes a measuring device having a function for automatic targeting of a retroreflective target and having an automatic target tracking functionality. In order in this case, even in the event of rapid and jerky movements, to keep the target in the “locked on” state and not to lose it from the field of view of the fine targeting detector, it is proposed to record images of the target in parallel by means of a camera (which is sensitive in the visible wavelength range) and, with the aid of image processing, to track movements of the target (or movements of objects which move concomitantly together with the target), and thereby to make it easier for the retroreflector to be found again and locked on again in the case of the target being lost from the “locked on” state.
Furthermore, the prior art discloses methods wherein the solid angles between the measuring device and a target object are determined with the aid of an image and image processing (in particular on the basis of a position of a target object determined in the recorded image and depending on a known or also detected image recording direction). Such methods are described e.g. in WO 2005/026767 or in WO 2006/053837.
Moreover, European patent application No. 10168771.3 describes a motorized and automatic alignment of a targeting device of a measuring device with a target point of a (retroreflectorless) target mark. In this case, the target mark (or a set of different target marks) is previously known and has a structure/shape (pattern, form, etc.) suitable for indicating the target point on the target mark. By means of an evaluation unit, in a detected image, the target mark is identified on the basis of the pattern and the position of the imaged target mark in the image is determined highly precisely. Depending on this determined position of the target mark or of a pattern correlated with the target mark in the image, the targeting device can then be precisely aligned.
One disadvantage of measuring devices according to the prior art is, on the one hand, the given requirement of manual fine alignment of the targeting device with a target point. This can have a limiting effect on the accuracy during the measurement of the solid angles, since this process is dependent on the care and ability of the user. Furthermore, manual accurate targeting of a target point is associated with a certain time expenditure which has an adverse effect on the user's productivity.
On the other hand, the automatic fine targeting in measuring devices according to the prior art is limited to defined targets. In this regard, the ATR fine targeting function described functions only for cooperative targets, such as e.g. prisms, and not for natural target points. Other methods of automatic fine targeting function only for defined patterns, e.g. for target marks whose forms are known to the system, and not for objects having a certain variability in their shape, e.g. church towers.